Method of preparing epoxy-glass prepregs

ABSTRACT

Epoxy resin compositions comprising a first epoxy resin having a relatively low epoxy equivalent weight of about 170 to 200 and an average functionality of 2 or less, a second epoxy resin having a relatively high epoxy equivalent weight of about 900 to 1900 and an average functionality of 2 or less, and a curing system. Glass fabric is impregnated with the epoxy resin composition and formed into laminates, which are especially useful in the manufacture of printed circuit boards.

This is a division of application Ser. No. 169,463 filed Aug. 5, 1971,now abandoned.

This invention relates to epoxy resin compositions. In its more specificaspect, this invention relates to epoxy resin compositions especiallyuseful in glass-base laminates exhibiting improved properties.

Glass-base laminates prepared from glass fabric impregnated with anepoxy resin are used in the manufacture of printed circuit boards. Thelaminate should exhibit, as a general rule, good mechanical, electricaland chemical properties as well as flame retardancy. Nonetheless, theselaminates must keep apace with advances in technologies utilizing themwhich, in turn, lends to more stringent property requirements.

Epoxy glass laminates, having a copper clad on one or both faces, aresubject to severe cleaning and etching processes. The laminate,therefore, must possess superior chemical and solvent resistance.Equally important is the peel strength which is the measure of bondstrength between the metal foil and laminate. This bond should besufficiently strong not only as measured at room temperature and for hotpeel at 125° C., but also after the solder float test with solder at500° F. and after continuous service at high temperatures of about 100°C.

In the manufacture of printed circuit boards, the laminate is subjectedto molten solder by solder floating or solder waving in order to connectthe electrical components to the board. If the laminate is defective, itmay show numerous cracks on the surface, and in extreme cases blistersmay form. This defect is known as "measling," which generally is causedby relatively low heat-resistance or high moisture absorption of thelaminates. These adverse properties are affected by the quality of thelaminating epoxy resins and, to some extent, by the manufacturingprocesses.

When electrical components are to be connected to the printed circuitboard, a plurality of holes are drilled in the laminate, which may beplated or soldered to provide electrical connections. Holes drilled withhigh speed carbide tipped drills must be smooth and clean and have gooddimensional tolerance so that electronic components can be inserted intothe holes and firmly soldered in place. If the epoxy resin compositionsets to a very high cross-linked density, forming a tight and densecross-linked lattice, the resin may cause rapid wearing of the drills.On the other hand, improperly balanced epoxy resin compositions thathave a low heat distortion temperature would give a "soft" laminate, andthe heat generated during drilling softens the drill dust which smearsthe holes or sticks to the drill. In severe cases, the excessive heatgenerated by drilling might degrade the surrounding resin to show adefect known as "halo" and separation of the metal foil from thelaminate around the holes might occur.

Efforts have been made to overcome the aforementioned problems ordefects by modifying the epoxy resin formulation or curing system, or byvarying the manufacturing operations. While some properties have beenimproved to the expected levels, it is often discovered that suchimprovements are at the expense of other properties. For example, epoxyresin having a high functionality, such as an epoxy novolac or tetrakis(hydroxyphenyl) ethane tetraglycidal ether, may be blended into thediglycidyl ether of bisphenol-A type epoxy resin to improve the elevatedtemperature performance and high distortion temperature and to reducethe drill smear of the laminate, but the resin results in laminatesexhibiting high rigidity, higher drill wear, and possibly low metal peelstrength and discoloration. Post curing of laminates made fromconventional epoxy resin often improves their properties; it also maycause warpage and surface non-uniformity. Similarly, modifying thecuring system may adversely affect metal foil adhesion, color of thelaminates, electrical properties, mechanical properties and hardness.

This invention has therefore as its object to provide improved epoxyresin compositions suitable for impregnating glass fabrics for use inthe manufacture of laminates, including metal clad laminates, whichexhibit superior electrical, physical and chemical properties.

According to this invention, there is provided an epoxy resincomposition comprising a blend of epoxy resins having substantiallydifferent epoxy equivalent weights and each having an averagefunctionality of 2 or less, but in no event is the functionality for aresin molecule greater than 2, and a curing sytem. The first epoxy resinhas a relatively short molecular chain length indicated by an epoxyequivalent weight of about 170 to 200, and is sometimes referred toherein as the blending resin. The second epoxy resin, sometimes referredto herein as the epoxy base resin, has a relatively long molecular chainlength indicated by an epoxy equivalent weight of about 900 to 1900, andmay be non-halogenated or halogenated (e.g., brominated). The epoxyresins are described herein below in greater detail.

In preparing the resin composition, the base resin and blending resinare admixed at room temperature in the ratios of about 70 to 40% byweight of the base resin to 30 to 60% by weight of the blending resin,and preferably 65 to 50% to 35 to 50%. Sufficient or adequateantimeasling is not attained if the resin compositions contain less thanabout 30% by weight of the blending resin. On the other hand, if morethan 60% of the blending resin is used, the finished laminate is toohard for general application and further may interfere with drilling.

It is well known that epoxy resins may be prepared as the reactionproduct of a halohydrin and a phenol. A widely used class of epoxyresins is typified by the reaction between epichlorohydrin andbis-(4-hydroxyphenyl-2,2 propane, the latter being commonly referred toas bisphenol A, and may be represented by the general formula: ##STR1##where n may have an average numerical value between 0 to about 7,depending for the most part on the relative proportions of tworeactants. Although the invention is described in detail with referencesto epichlorohydrin and bisphenol A, it should be understood that otherhalohydrins and polyhydric phenols may also be used. Such epoxy resinsare sold under the tradenames of Epon by Shell Chemical Corporation,Araldite by Ciba Company, Epi-Rez by Celanese Corporation, ERL byBakalite Company, and D.E.R. by Dow Chemical Company.

The epoxy blending resin is prepared by reacting epichlorohydrin andbisphenol A in the presence of sodium hydroxide as a scavengeringcatalyst, or other suitable alkali metal catalyst. The proportion ofreactants may range from about 2 to 10 moles of epichlorohydrin per moleof bisphenol A. The resulting epoxy resin is characterized by an epoxyequivalent weight of 170 to 200, wherein the value of n in the abovegeneral formula is between 0 to 3, and an average functionality of 2 orless, usually 1.7 to 2, but in no event is the functionality for a resinmolecule greater than 2. Such resins are available on the market, forexample, as Epon 828, Epon 826, Epon 825, D.E.R. 332, D.E.R. 331,Epi-Rez 510 and Epi-Rez 5108.

The epoxy base resin is prepared by reacting (1) the diglycidyl ether ofbisphenol A with (2) a calculated amount of bisphenol A or othersuitable polyhydric phenol such as dihydroxy diphenyl, bisphenol A,hydroquinone and resorcinol. Where desired, a halogenated bisphenol suchas tetrabromobisphenol A may be substituted for the bisphenol A. Theepoxy equivalent weight of the resulting resin depends largely upon theproportions of reactants, which may range from about 20 to 50% by weightof bisphenol A. The resulting epoxy resin base desirably has an epoxyequivalent weight of 900 to 1900 and an average functionality of notmore than 2. Generally the epoxy equivalent weight for the base resin islower when bisphenol A is used than when tetrabromobisphenol A is used.

In the preferred embodiment, the epoxy base resin is prepared by thefusion process described in U.S. Pat. No. 3,477,990, because of itssimplicity and control of molecular weight. In the fusion process, epoxyresin of digycidyl ether of bisphenol A is combined with a calculatedamount of bisphenol A or tetrabromobisphenol A and an organo-phosphoniumhalide, such as methyl triphenyl phosphonium iodide, and reacted at 150°to 200° C. The epoxy resin containing the phosphonium halide catalyst issold under the tradename Epon 829 in solution form containing about 3.5%by weight of xylene-oxital solvent mixture. Other precatalyzed epoxyresins which are also useful and sold as a liquid include D.E.R.7030.6and ARALDITE 9410. Where desired, other dihydric phenols or biphenolscan be substituted for the bisphenol A or tetrabromobisphenol A asdescribed in the aforesaid patent. The epoxy base resins prepared by thefusion process have an epoxy equivalent weight ranging from as low as500 to as high as 7000 depending on the calculated quantity of bisphenolused. For purposes of this invention, the epoxy equivalent weight forthe epoxy base resin should range from about 900 to 1,400 when bisphenolA is used and from about 1200 to 1900 when tetrabromobisphenol A isused.

The curing system for the epoxy compositions of this invention includesa cross-linking agent and a promoter, which preferably are dissolved ina suitable solvent and then admixed with the epoxy resins. Dicyandiamideis especially useful as the cross-linking agent for epoxy resincompositions which are anti-measling. In the preferred embodiment, about2.5 to 5 parts by weight based on the total weight of the resincomposition of dicyandiamide dissolved in dimethyl formamide is used asthe cross-linking agent, but other suitable curing systems includediamino-diphenyl-sulfone as the cross-linking agent together with borontrifluoride mono-ethylamine complex as the promotor, meta-phenolenediamine as the cross-linking agent, and organic anhydride cross-linkingagents such as hexahydrophthalic anhydride or chorendic anhydride whichmay be used with or without a promoter. When dicyandiamide is used, itis preferable to use a tertiary amine as the promoter, usually in anamount of about 0.3 to 0.8 parts by weight per 100 parts of resinsolids. These tertiary amines may include, for example,benzyldimethylamine, 2,4,6-tris(di-methyl-amino methyl) phenol,hydroquinone-blocked triethylene-di-amine, 1-methyl imidazole and otherimidazoles.

The resin mixture and curing system may be diluted with a suitablesolvent or diluent to render the resin composition especially useful asan impregnating composition. Preferably, the final resin composition has40 to 60% resin solids, a viscosity of 15 to 50 centipoises and aspecific gravity of 0.990 to 1.150. Suitable solvents include, forexample, ethylene gylcol monomethyl ether (methyl cellosolve), acetoneand other ketones such as methylethyl ketone.

The invention is further illustrated by the following examples, in whichthe quantities are stated in parts by weight unless otherwise indicated.

EXAMPLE I

60 parts of an epoxy base resin having an epoxy equivalent weight of 950prepared by the fusion process described above from Epon 829 andbisphenol A was dissolved in 6 parts of methyl Cellosolve and 36 partsof acetone and then blended with 40 parts of Epon 828 as the epoxyblending resin. The resulting resin blend had a solids content of 70%and a specific gravity of 1.060 and a viscosity of 450 centipoises.Three parts of dicyandiamide was dissolved in 15 parts of dimethylformamide and 9 parts of methyl Cellosolve. The dicyandiamidecross-linking solution was mixed with the resin blend, and 0.5 parts ofbenzyl dimethylamine as a promoter was added. Twenty eight parts ofacetone was added to dilute the final resin composition to 53% solidscontent. Six hours after the addition of the promoter, the final resincomposition had a specific gravity of 1.010 and a 170° C. stroke cure of210 to 220 seconds. The pot life of the final resin composition wasabout 3 days at 20° C. or below. Its storage life without the benzyldimethylamine promoter was more than 3 months.

A woven glass fabric of 7 mils thickness prefinished with silane varnishwas dipped in the epoxy resin composition to form a prepreg (a partiallycured epoxy resin impregnated glass fabric). Impregnation of the fabricwas adjusted in such a way that when the wet impregnated sheet was driedat 155° C. in an oven to advance the resin, 10 to 16% by weight of theresin, based on the combined weight of the resin composition and fabric,was pressed out of the prepreg in a hydraulic press at 170° C. and undera pressure of 1000 psi but that before squeeze out the weight increaseof the fabric due to the resin pick-up was 66 to 73%.

A plurality of sheets were cut from the prepreg and stacked with oneounce copper foil. The layup was pressed at an elevated temperature andpressure by conventional practice to form a laminate.

Conventional tests for laminates of this type were conducted, and thetests results are set forth in the following table:

                                      TABLE I                                     __________________________________________________________________________    TESTS FOR LAMINATES                                                                              Charac-                                                                             NEMA                                                             Condition                                                                            teristics                                                                           Requirements                                                                          Methods                                      __________________________________________________________________________    Thickness          0.062"                                                     Water Absorption                                                                          D24/23 0.072%                                                                              0.35 max.                                                                             NEMA LI--1-10.12                             Flexural Strength                                                                         L      82 000                                                                              60 000 min.                                                                           ASTM D790                                                C      62 000                                                                              50 000 min.                                                                           ASTM D790                                    Dissipation Factor                                                                        D24/23 0.023 0.035max.                                                                             ASTM D150                                    Dielectric Constant                                                                       D24/23 4.43  5.40 max.                                                                             ASTM D150                                    Surface Resistance                                                                        C96/35/90                                                                            50,000                                                                              1,000 min.                                                                            ASTM D257                                    Volume Resistance                                                                         C96/35/90                                                                            11.6 × 10.sup.6                                                               1 × 10.sup.6 min.                                                               ASTM D257                                    Peel Strength (2 oz.)                                                                     A      13.5  11.0 min.                                                                             NEMA LI--1-10.12                             (Elevated)  60' at 125° C                                                                 13.2  --      NEMA LI--1-10.12                             Solder Flost Blistering                                                                   500° F                                                                        >180 sec.                                                                           20 sec. min.                                                                          NEMA LI--1-10.11                             __________________________________________________________________________

In addition, a 15 psig. steam pressure cooking test was conducted forthe purpose of determining the anti-measling property of the laminates.According to this test, samples measuring 2 × 3 inches were placed in anautoclave just above the water line. The water was heated to 250° F. ata constant steam gauge pressure of 15 psi. The samples were held undersuch steam pressure; and every 1/2 hour a piece was taken out to checksolder float blistering for 60 seconds. The sample withstood a cookingtime of usually more than three hours without showing any measlingdefect after 60 seconds of solder floating. The specification for thistest requires that tests specimens show no measling or blistering aftertwo hours of steam pressure cooking. General purpose epoxy resins usedin glass laminates typically show measling or blistering after onlyone-half hour of pressure cooking time.

EXAMPLE II

Substantially the same resin composition as described in Example I wasused in preparing laminates except that 75 grams of the base resin wasadmixed with 25 grams of the blending resin. Laminates prepared fromthis composition showed measling after 60 seconds solder floating whenthe laminates had been subjected to pressure cooking test for two hours.According to the test specification there should be no measling aftertwo hours of pressure cooking. Failure clearly illustrated theimportance of the ratio of resins in attaining anti-measling.

EXAMPLE III

Sixty three parts of brominated epoxy resin base having an epoxyequivalent weight of 1400 and a bromine content of 26% by weightprepared by the fusion process described above from Epon 829 and amixture of tetrabromobisphenol-A and bisphenol A, was dissolved in 7parts of methyl Cellosolve and 37 parts of Epon 828. The resulting resinblend had a bromine content of 16.4% by weight, a specific gravity of1.148 and a viscosity of 350 centipoises based on a solids content of70%. The curing system of Example I was admixed with the resin blend,and glass laminates were made by the conventional process.

Test results on the laminates is shown in the following table:

                                      TABLE II                                    __________________________________________________________________________    LAMINATE TESTS RESULTS                                                                                       Charac-                                                                              NEMA                                                         Condition teristics                                                                            Requirements                                                                          Method                          __________________________________________________________________________    Thickness                      0.062"                                         Water Absorption     D24/23    0.073% 0.35 max.                                                                             NEMA LI--1-10.12                Solvent Resistance:                                                            Trichloroethylene Absorp.                                                                         7 min. at 190° F.                                                                1.5%                                            Trichloroethylene Surface     OK                                              50% Methylene Chloride Absorp.                                                                    30 min. at 75° F.                                                                1.5%                                            50% Methylene Chloride Surface                                                                              OK                                             Flexural Strength    L         86,500 60,000 min.                                                                           ASTM D790                                            C         63,500 50,000 min.                                                                           ASTM D790                       Dielectric Constant  D24/23    4.49   5.4  max.                                                                             ASTM D150                       Dissipation Factor   D24/23    0.022  0.035 max.                                                                            ASTM D150                       Dielectric Breakdown D48/50    S/T 76 30 Kv min.                                                                            ASTM D229                                                      S/S 68 30 Kv min.                                                                            ASTM D229                       Surface Resistivity  C96/35/90 72,000 1,000 min.                                                                            ASTM D257                       Volume Resistivity   C96/35/90 10 × 10.sup.6                                                                  1 × 10.sup.6                                                                    ASTM D257                       Peel Strength (2 oz.)                                                                              A         13.0   11.0 min.                                                                             NEMA LI--1-10.12                                     20 sec. at 500 ° F                                                               12.8   11.0 min.                               Peel at elevated temperature                                                                       60 min. at 125° C                                                                13.0   11.0 min.                               Seconds to blister   500° F                                                                           180 sec.                                                                             20   min.                               15 psi Pressure Cooking Test                                                                       250° F                                                                           3.0 hrs.                                                                             --      As in Example I                 Fire Retardancy      sec./inch 1-0 sec/0.75"                                                                        15 sec/1"                                                                             NEMA LI--1-10.20                Drill Test (0.042" carbide drill at                                           200 RPM)                                                                       No. of holes drilled          17,000                                          Smearing                      None                                            Drill wear                    1.43%                                          __________________________________________________________________________

The solvent resistance of the laminates was tested in the laboratory bythe following methods:

(a) Trichloroethylene Test--The copper foil was removed by etching withammonium presulfate from six sample pieces, which were cleaned, driedand carefully weighed and hung directly above boiling trichloroethylenein a closed tank. After two minutes duration in the vapor, the sampleswere taken out one at a time at one minute intervals and immediatelyweighed. The results are shown in Table III

                  TABLE III                                                       ______________________________________                                        SOLVENT ABSORPTION                                                            Time In                                                                       Trichloroethylene Vapor                                                                        Percent Absorption                                           ______________________________________                                        2 min.           1.012%                                                       3 min.           1.036%                                                       4 min.           1.074%                                                       5 min.           1.131%                                                       6 min.           1.157%                                                       7 min.           1.142%                                                       ______________________________________                                    

The absorption of trichloroethylene was substantially below therequirement. The samples were examined at 4 hour and 24 hour periods. Nosurface deterioration was observed on the etched side of the laminatesand the unclad sides showed only slight surface undulation.

(b) Methylene Chloride Test--The samples were prepared the same as forthe trichloroethylene test. They were immersed in 50:50 methylenechloride-methyl alcohol solution at room temperature and taken out oneat a time at five minute intervals, reweighed and examined. Solventabsorption is shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        SOLVENT ABSORPTION                                                            Time of Immersion                                                                              Percent Absorption                                           ______________________________________                                         5 min.          0.495%                                                       10 min.          0.792%                                                       15 min.          0.894%                                                       20 min.          1.175%                                                       25 min.          1.288%                                                       30 min.          1.246%                                                       ______________________________________                                    

The solvent absorption was well below the requirement, and the uncladsurface of the laminates showed even less undulation affected by theethylene chloride, which is a stronger solvent, than bytrichloroethylene.

The drill test was made with an automatic drilling machine using a new0.042 inch diameter carbide drill at a speed of 20,000 RPM. A stack ofthree sample laminates with 1 oz. copper foil clad on both sides of thelaminates were clamped firmly on the drilling table and three holes weredrilled through the three laminates. Drilling was automaticallycontinued, and after 5712 drillings or a total of 17136 holes, the drillshowed a diameter reduction, near the tip, of 0.0006 inch or 1.43%. Boththe drill points and cutting edges showed normal wear. Samples of epoxyglass laminates, clad on both sides with copper, made with generalpurpose epoxy resin were tested under the same condition, and thecarbide drill was broken after 2105 drillings or a total of 6315 holes.The last portion of the holes were slightly irregular and evidenced somesmearing.

It will be observed from the foregoing examples and tests that laminatesmade from prepregs impregnated with epoxy resin compositions of thisinvention exhibit excellent properties, especially non-measling,non-smearing upon drilling, and solvent resistance.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A method for preparing prepregs for use in themanufacture of epoxy glass circuit board substrate laminatescharacterized by improved antimeasling, solvent resistance andanti-smearing upon drilling, which comprises a) impregnating a glassfabric with an epoxy resin composition comprising a first epoxy resin ofdiglycidy ether of bisphenol A having an epoxy equivalent weight of 170to 200 and an average functionality of not more than 2, and a secondepoxy resin comprising the reaction product of diglycidyl etherbisphenol A epoxy resin and a polyhydric phenol selected from the groupconsisting of bisphenol A and tetrabromobisphenol A, said resultingsecond epoxy resin having an epoxy equivalent weight of 900 to 1900 andan average functionality of not more than 2, the proportions of saidfirst resin ranging from about 30 to 60% by weight of resin solids andof said second resin from about 70 to 40% by weight of resin solids, acuring system, and a suitable solvent, and b) passing the epoxyimpregnated glass fabric through an oven maintained at an elevatedtemperature to partially advance said epoxy resin and to remove saidsolvent.
 2. A method for preparing a prepreg according to claim 1wherein said polyhydric phenol is bisphenol A and the epoxy equivalentweight of said second epoxy resin is 900 to
 1400. 3. A method forpreparing a prepreg according to claim 1 wherein said polyhydric phenolis tetrabromobisphenol A and the epoxy equivalent weight of said secondepoxy resin is 1200 to
 1900. 4. An method according to claim 1 whereinsaid curing system includes about 2.5 to 5 parts by weight per 100 partsof resin solids of dicyandiamide as a cross-linking agent and a tertiaryamine as a catalyst.
 5. An method according to claim 1 wherein theproportions of said first resin ranges from about 35 to 50% by weightand of said second resin from about 65 to 50% by weight.
 6. A method forpreparing prepregs for use in the manufacture of epoxy glass circuitboard substrate laminates characterized by improved anti-measling,solvent resistance and anti-measling upon drilling, which comprises a)impregnating a glass fabric with an epoxy resin composition comprising afirst epoxy resin of diglycidyl ether of bisphenol A having an epoxyequivalent weight of 170 to 200 and an average functionality of not morethan 2, a second epoxy resin comprising the reaction product ofdiglycidyl ether bisphenol A epoxy resin and bisphenol A, said resultingsecond epoxy resin having an epoxy equivalent weight of 900 to 1400 andan average functionality of not more than 2, the proportions of saidfirst resin ranging from about 35 to 50% by weight of total resin solidsand of said second resin from about 65 to 50% by weight of total resinsolids, about 2.5 to 5 parts by weight per 100 parts of total resinsolids of dicyandiamide as a cross-linking agent, a tertiary amine as acatalyst, and a suitable solvent whereby the resultant compositionincludes about 40 to 60% by weight of total resin solids, and b) passingthe epoxy impregnated glass fabric through an oven maintained at anelevated temperature to partially advance said epoxy resin and to removesaid solvent.
 7. A method for preparing prepregs for use in themanufacture of epoxy glass circuit board substrate laminatescharacterized by improved anti-measling, solvent resistance andanti-smearing upon drilling, which comprises a) impregnating a glassfabric with an epoxy resin composition comprising a first epoxy resin ofdiglycidyl ether of bisphenol A having an epoxy equivalent weight of 170to 200 and an average functionality of not more than 2, a second epoxyresin comprising the reaction product of diglycidyl ether bisphenol Aepoxy resin and tetrabromobisphenol A, said resulting second epoxy resinhaving an epoxy equivalent weight of 1200 to 1900 and an averagefunctionality of not more than 2, the proportions of said first resinranging from about 35 to 50% by weight of total resin solids and of saidsecond resin from about 65 to 50% by weight of total resin solids, about2.5 to 5 parts by weight per 100 parts of total resin solids ofdicyandiamide as a cross-linking agent, a tertiary amine as a catalyst,and a suitable solvent whereby the resultant composition includes about40 to 60% by weight of total resin solids, and b) passing the epoxyimpregnated glass fabric through an oven maintained at an elevatedtemperature to partially advance said epoxy resin and to remove saidsolvent.